Continuous electrolytic pickling and tin plating of steel strip



March 30, 1954 VQNADA 2,673,836

CONTINUOUS ELECTROLYTIC PICKLING AND TIN PLATING OF STEEL STRIP Filed Nov. 22, 1950 2 Sheets-Sheet l [0W/N f. Vo/mm,

' l Mar/w 75mm? I lll l l ll l I ll- I E. VONADA ELECTROLYTIC PICKLING AND TIN PLATING OF STEEL STRIP March 30, 1954 Filed Nov. 22, 1950 [am/v f. Val 404,

Patented Mar. 30, 1954 CONTINUOUS ELECTROLYTIC PICKLING AND TIN PLATING OF STEEL STRIP Edwin E. Vonada, Cleveland, Ohio, assignor to United States Steel Corporation, a corporation of New Jersey Application November 22, 1950, Serial No. 196,975 4 Claims. (01. 204-28) This invention relates to the continuous electrolytic pickling and tin plating of steel strip and is a continuation-in-part of my copending application Serial No. 110,365, filed August 15, 1949, now abandoned. In the continuous method of electrolytic tin plating now in common use steel strip which has been hot rolled, pickled, cold rolled, cleaned, annealed in a reducing atmosphere and in most cases temper rolled is supplied to the tin line. This strip has a thin coating of oxide thereon which must be removed prior to plating. The usual practice is to contiuously pass the strip from an uncoiler through a cleaning tank which removes oil therefrom and then through a pickling bath which removes the thin oxide coating. The cleaned and pickled strip then passes to the electrolytic tin coating tank.

I have found that by simultaneously pickling and applying a flash coating to the strip instead of merely pickling the strip, a much better finished product is obtained. This is done by using a combination of tin anodes and silicon containing ferrous anodes in a sulphuric acid bath in somewhat the same manner as disclosed in the patent to Fink et al. No. 1,927,116, dated September 19, 1933. However, the method disclosed therein is not broadly applicable for applicants purpose since certain conditions not disclosed therein must be adhered to before getting the desired results. I have also found that the current density disclosed in the Fink et al. patent will not give the tight inherent flash coating required for subsequent electrotinning. Instead, it is necessary to use several times the disclosed current density.

It is therefore an object of my invention to provide a continuous method of pickling, flash coating and electrotinning steel strip.

Another object is to provide a continuous method of simultaneously pickling and applying a flash tin coat to steel strip suitable for electrotinning.

These and other objects will be more apparent after referring to the following specification and attached drawings, in which:

Figure 1 is a schematic longitudinal section through the electrolytic tank;

Figure 2 is a horizontal sectional view taken on the line II-II of Figure 1; and

Figure 3 is a schematic layout of an electrolytic chamber 4 in the lower part thereof. The chamber i is separated from the top part of the tank by means of insulation 5 and division plates 8. A rubber covered sink roll it is positioned in the chamber 4 which has an inlet 2 for the electrolyte. An insulating barrier [4 extends from the center division plate 8 to a deflector plate it at the top of the tank and divides the top of the tank into two chambers l8 and 28 which are insulated from each other. A constricted passageway 22 in chamber [8 is insulated from the rest of the tank by means of insulation 25 at the bottom thereof. Fastened to the outside of passageway 22 and to the sides of tank 2 are deflector plates 26. Within the passageway 22 are removable electrode liners 28 which may be adjusted for movement toward and away from each other in any suitable manner. Each of the electrode liners 28 is preferably made up of a silicon containing anode 283 at each end thereof near the edge of the strip S, a tin anode 28'1 adjacent each anode 28S and then silicon containing anodes 23S alternating with tin anodes 2ST. For best results the anodes 288 should have half the surface area of anodes 28S and the anodes 281 should have one-tenth the surface area of anodes 28S. Instead of the above arrangement, the electrode liners 28 may be made up of silicon containing anodes and tin anodes arranged in a checkerboard fashion. The silicon containing anodes and tin anodes also be arranged alternately along the length of the strip. In any event in order for the method to operate satisfactorily it is necessary to have the tin and silicon containing anodes arranged in an alternate pattern and for the surface area of the tin anodes to be between 8% and 12% of the total anode area. Best results are obtained when the surface area of the tin anodes is 9% of the total anode area and with the arrangement described above. However, the relationship between adjacent tin and silicon bearing anodes can vary in the same proportion as their total areas; that is, the surface area of a tin anode may be between 8% and 12% of the combined surface area of a tin and silicon bear hug anode. The width of anodes 233' may be as great as five inches although it is preferred to not exceed a width of four inches. Insulation 3t separates the liners 28 from the passageway 22. A restricted passageway 32L in chamber 28 is insulated from the rest of the tank by insula tion 34. Deflector plates ttgare fastened to the outside of passageway 32 and to the sides of tank 2. Renewable electrode liners 38 made up in the same manner as liners 28 are adjustably mounted within the passageway 32 and are separated therefrom by insulation 40. An entry conductor roll 42 is mounted above chamber it! and a delivery conductor roll 44 is mounted above the chamber 20. A hood 45 surrounds the rolls 42 and 44 and the top of the tank 2 for collecting the fumes from the cleaning operation. A conduit 48 leads from the hood G to an exhaust fan, not shown. The negative terminal of a generator 50 is connected to the entry conductor roll 42 and its positive terminal is connected to the electrode liners 28. A second generator 52 has its positive terminal connected to the roll M and its negative terminal connected to the electrode liners 38.

The strip S passes from an uncoiler 55 to a cleaning tank 56 and enters the tank 2 over the entry conductor roll 42 and passes downwardly between the electrode liners 28 which are so ar- If ranged that the clearance between the electrodes and the strip will give the most effective cleaning. The strip then passes under the sink roll it upwardly between the electrode liners 38 and over the exit roll 44. Hold down rolls 54 are provided at each conductor roll to insure good contact between the rolls and the strip. The hold down roll for the exit conductor roll also prevents electrolyte drag out.

The electrolyte is circulated by means of a pump 64 which delivers it from a reservoir 66 to the chamber 4 through the opening 12. Insulation 68 is provided to prevent grounds and current leakage from the reservoir 66. The electrolyte passes upwardly through passageways 22 and 32 and over the surface of the strip S at high velocity. The electrolyte soiution then flows over the top of the passageways and cas cades downwardly over the plates and 56 which aid in releasing the entrained gases in the electrolyte. Deflector plates IE confine the electrolyte within the tank. The electrolyte leaves the tank through conduits '10 which lead to the reservoir 66. Means for cleaning and conditioning the electrolyte are preferably located in the reservoir 66. The above described apparatus is more particularly described and claimed in my copending application, Serial No. 725,056, filed January 29, 1947, now Patent No.

2,556,017. From the exit roll 44 the strip S that much more equipment is provided in the tinning line which is not illustrated or described since it has no bearing on the present invention.

I have found that by using a ferrous body containing between 13% and 18% silicon in the silicon containing anode, I obtain the desired corrosion resistance and physical properties as well as acceptable electric conductivity characteristics. A ferrous alloy found suitable for such anodes contains substantially .'75% to carbon; 30% to .70% manganese; and 14.25% to 14.50% silicon with the usual amounts of residual elements. Such anodes have a comparative insolubility under anodic corrosion, an adequate electric conductivity, favorable hydrogen overvoltage characteristics of corroded surface in the cleaning bath, and freedom from chemical contamination of the bath by the products of anodic decomposition. In place of using silicon iron alloys, a ferrous body having its surface impregnated with a similar amount of silicon may be used. Apparently these anodes operate satisfactorily because of the formation of a film of hydrated silicon oxide on the surface of the anodes under conditions of continuous electrolytic cleaning. This coating is electrically conducting and allows the current to pass, but prevents contact between the metal of the anode and the solution.

The silicon bearing anodes described above are used in conjunction with tin anodes to apply a flash coating of tin to the strip S while simultaneously cleaning it. The overvoltage on the tin anodes ishigher than on the iron anodes so that the current has less resistance to overcome in the case of the iron electrodes, with the result that the less expensive iron electrodes are dissolved in preference to the tin. The overvoltage on the tin anodes is also higher than on my silicon bearing anodes and the process of electrolytic cleaning and tin coating is especially satisfactory when using such silicon bearing anodes in conjunction with tin anodes in a 5% to 15% sulphuric acid solution. Under operating conditions the sulphuric acid will contain some ferrous and ferric sulphate. The bath temperature is preferably maintained between and 150 F. and the current density must be between 150 and 250 amperes per square foot.

In carrying out my process, tank 4 is prefer-- ably filled with a 10% solution of sulphuric acid maintained at F. and containing about 1% ferrous sulphate. Generators 58 and together with pump 64 are placed in operation and the strip S is started through the apparatus.

Operating conditions are then adjusted to provide for a strip travel speed of between 480 and 600 feet per minute with the current density over the total exposed area of electrode linings being adjusted to between and 250 amperes per square foot with the voltage maintained below 12 volts. Strip passing through the apparatus under these operating conditions is thoroughly pickled. but is not flash coated until the tin concentration in the electrolyte is built up. When it is desired to flash coat the strip from the beginning of the operation, a per centage of stannous sulphate found desirable for the specific cleaning conditions used is introduced in the electrolyte. By way of example, it has been found that the addition of substantially 2% SnSOq. to a 10% solution of sulphuric acid produces an adequate flash tin coating. Under these conditions a fine layer of tin is coated on the strip with a good bond between the tin and steel. The strip S with the flash coat of tin thereon then passes to the electrolytic coating tank 58 where the desired weight of tin coating is applied.

W-hile one embodiment, of my invention has been shown and described it will be apparent that other adaptations and modifications may be made without departing from the scope of the following claims.

I claim: x

1. The method of continuously and simultaneously pickling and applying a flash coating of tin to a steel strip having a thin cotaing of oxide thereon which comprises passing the strip through a 5% to 15% sulphuric acid solution having a plurality of anodes therein, part of anodes being madefof tin and the remainder. be

ship across the width of the strip with one adjacent each edge of the strip and tin anodes between each pair of silicon containing anodes, the total surface area of the tin anodes being between 8% and 12% of the total anode area, and maintaining a current density between 150 and 250 amperes per square foot of anode area.

2. The method of continuously electrolytic pickling and tin plating steel strip having a thin cotaing of oxide thereon which comprises cleaning the strip and applying a flash coating of tin thereto by passing it through a 5% to sulphuric acid solution having a plurality of anodes therein, part of said anodes being made of tin and the remainder being made of ferrous metal containing between 13% and 18% silicon, said silicon containing anodes being arranged in spaced apart relationship across the width of the strip with one adjacent each edge of the strip and tin anodes between each pair of silicon containing anodes, the total surface area of the tin anodes being between 8% and 12% of the total anode area, maintaining a current density between 150 and 250 amperes per square foot of anode area, and then passing the steel strip with the flash coating thereon through an electrolytic tin coating tank.

3. The method of continuously and simul-- taneously pickling and applying a flash coating of tin to a steel strip having a thin coating of oxide thereon which comprises passing the strip through a 5% to 15% sulphuric acid solution having a plurality of anodes therein, part of said anodes being made of tin and the remainder being made of ferrous metal containing between 13% and 18% silicon, said silicon containing an odes and said tin anodes being arranged in an. alternate pattern, the total surface area of the tin anodes being between 8% and 12% of the total anode area, and maintaining a current density between and 250 amperes per square foot of anode area.

4. The method of continuously electrolytic pickling and tin plating steel strip having a thin coating of oxide thereon which comprises clean ing the strip and applying a flash coating of tin thereto by passing it through a 5% to 15% sulphuric acid solution having a plurality of anodes therein, part of said anodes being made of tin and the remainder being made of ferrous metal containing between 13% and 18% silicon, said silicon containing anodes and said tin anodes being arranged in an alternate pattern, the total surface area of the tin anodes being between 8% and 12% of the total anode area, maintaining a current density between 150 and 250 amperes per square foot of anode area, and then passing the steel strip with the flash coating thereon through an electrolytic tin coating tank.

EDWIN E. VONADA.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date Re.16,598 Hambuechen Apr. 19, 1927 Re. 19,773 Dunn Dec. 3, 1935 1,599,284 Proctor Sept. 7, 1926 1,927,116 Fink et a1. Sept. 19, 1933 1,979,996 Phillips Nov. 6, 1934 2,274,963 Hopper Mar. 3, 1942 2,370,986 Nachtman Mar. 6, 1945 2,535,966 Teplitz Dec. 26, 1950 2,556,017 Vonada June 5, 1951 OTHER REFERENCES Transactions of the Electrochemical Society, vol, 66, 1935, pages 381-391. 

1. THE METHOD OF CONTINUOUSLY AND SIMULTANEOUSLY PICKLING AND APPLYING A FLASH COATING OF TIN TO A STEEL STRIP HAVING A THIN COATING OF OXIDE THEREON WHICH COMPRISES PASSING THE STRIP THROUGH A 5% TO 15% SULPHURIC ACID SOLUTION HAVING A PLURALITY OF ANODES THEREIN, PART OF SAID ANODES BEING MADE OF TIN AND THE REMAINDER BEING MADE OF FERROUS METAL CONTAINING BETWEEN 13% AND 18% SILICON, SAID SILICON CONTAINING ANODES BEING ARRANGED IN SPACED APART RELATIONSHIP ACROSS THE WIDTH OF THE STRIP WITH ONE ADJACENT EACH EDGE OF THE STRIP AND TIN ANODES BE- 